The present invention relates to a fuel cell electrode and to a method for manufacturing the electrode. The electrode is designed to be utilized with an ionomer membrane in a fuel cell assembly.
Fuel cells are primary generators of electrical power. Fuel cells are similar to chemical batteries, such as lead or alkaline batteries, in that electricity is generated from a reaction of fuel and an oxidant. Unlike chemical batteries, however, the fuel and oxidant in a fuel cell are continuously resupplied. Thus, fuel cells never have to be electrically recharged. The cells require only a new supply of fuel and oxidant for continued operation.
A fuel cell includes two chambers, a first chamber for containing the fuel, usually hydrogen, and a second chamber for containing an oxidant, usually oxygen or an oxygen-rich gas such as air. The fuel and oxidant chambers sandwich two electrodes which, in turn, surround an electrolyte. Hydrogen molecules are adsorbed at one electrode, an anode, to break the hydrogen molecular bonds, creating hydrogen ions and free electrons. The electrons, as will be discussed, flow from the anode to a load device, such as a light bulb, and flow on to a second electrode, a cathode. Oxygen molecules are adsorbed at the cathode, and the hydrogen ions, migrating through the electrolyte, react with the oxygen molecules at the cathode in a reduction reaction to produce water. The adsorption of the hydrogen and oxygen molecules is stimulated by a catalyst layer serving as an interface between each of the two electrodes and the electrolyte. A potential difference existing between the hydrogen and oxygen electrodes, anode and cathode, respectively, thus creates an electrical current. Once the electrons reach the cathode, they are consumed by the reduction reaction.
Fuel cells which use an ionomer membrane as the electrolyte have a significant advantage over fuel cells which use a liquid electrolyte system. Liquid electrolyte systems, such as alkaline and phosphoric acid systems, require complex subsystems to assure the purity of the electrolyte, its continuous circulation and, most importantly, that a fixed three-phase boundary is maintained. The three-phase boundary is an interface at which the reactant gases, the electrode catalyst and the electrolyte meet. Unless precise controls are maintained, a liquid electrolyte can flood the three-phase boundary and thereby prevent the reactant gases from efficiently reaching the catalyst.
Ionomer membranes eliminate the need for complex electrolyte subsystems and the precise controls otherwise necessary to maintain a fixed, three-phase boundary in a fuel cell. There are many ionomer membranes currently being investigated for fuel cell applications. The most typical ionomer membrane for this application is a proton exchange membrane. In a proton exchange membrane, acid groups, bonded within the membrane, facilitate the transit of protons from one side of the membrane to the other. Hydrogen ions are the typical species of proton which are transported using a proton exchange membrane. The transport of hydrogen ions within the membrane proceeds via a complex mechanism, including a Grothius chain-type mechanism. Therefore, water molecules are normally required for hydrogen ion transport. If the ionomer membrane is not sufficiently hydrated, reduced hydrogen ion transfer will occur, and the fuel cell's performance will degrade. In extreme cases, dehydration of the membrane at elevated temperatures can lead to cracking of the membrane and loss of its ion-conducting capability. Recently, it has been reported that doping the ionomer membrane with a heteropoly acid can help alleviate the dehydration problem by substituting non-volatile acid groups for water in the membrane. The following is a sampling of the proton exchange membranes currently under investigation for fuel cell use: (1) the Ballard/Dow membrane, manufactured by Ballard Power Systems of Vancouver, Canada; (2) the Naflon series of membranes, manufactured by DuPont Chemical Company of the United States; (3) the Flemion series of membranes, manufactured by Asahi Glass Co. of Japan; and (4) the DAIS membrane, manufactured by DAIS Company of Palm Harbor, Fla. The membranes vary in thickness and in chemical composition.
One type of ionomer membrane, e.g., a DuPont product known as Nafion 117.RTM., eliminates a need for complex electrolyte subsystems and the precise controls otherwise necessary to maintain a fixed, three-phase boundary in a fuel cell. Nafion 117.RTM. is a proton exchange type of ionomer membrane. Acid groups, bonded within the membrane, facilitate the transit of protons from one side of the membrane to the other. Hydrogen ions are the typical species of proton which are transported using a proton exchange membrane. The transport of hydrogen ions within the membrane proceeds via a Grothius chain mechanism and, therefore, four to six water molecules are required for each hydrogen ion transported. If the ionomer membrane is not sufficiently hydrated, reduced hydrogen ion transfer will occur, and the fuel cell's performance will degrade. In extreme cases, dehydration of the membrane at elevated temperatures can lead to cracking of the membrane and loss of its ion-conducting capability.
Ionomer membranes have a second, potentially serious drawback. The membranes are comprised of essentially smooth, flat two-dimensional materials. The membranes, therefore, present an inherent impediment to extending a three-phase boundary into a third spatial dimension, which is unlike liquid electrolyte systems. In addition, ionomer membranes have a propensity to expand when wet. Consequently, it is difficult to attach a high-surface area electrode onto the ionomer membrane surface in order to extend the three-phase boundary. Thus, while ionomer membranes represent an advance over liquid electrolyte systems for fuel cell purposes, they present their own unique problems in designing a practical fuel cell assembly.
Some fuel cell designers have chosen to use a hydrophobic electrode in conjunction with the membrane. A hydrophobic electrode will tend to retain water within the membrane and thereby reduce the overall loss of water from fuel cell assembly during operation. Hydrophobic fuel cell electrodes are typically composed of high surface area carbon particles, a graphite cloth backing layer and Teflon.RTM.. Teflon.RTM. particles, dispersed in an aqueous suspension, and the carbon particles are mixed. The mixture is applied to the graphite cloth backing layer. The electrode is then heated in order to sinter the Teflon.RTM. particles. The sintered Teflon.RTM. particles are hydrophobic and also serve to provide channels for the reactant gases to reach the three-phase boundary. The electrical current produced at the catalyst layer flows via the carbon particles to the graphite cloth backing and then to a current collector. A typical hydrophobic electrode is the Prototech.RTM. electrode, manufactured by the E-Tech Company in Massachusetts.
Traditional fuel cell technology has relied upon hydrophobic electrodes based on a belief that hydrophobicity was necessary to prevent flooding of pores within the electrode, especially on the oxidant side of a fuel cell where product water is produced, and hence to maintain optimum contact between the three phases of the structure.
Unfortunately, hydrophobic electrodes also present significant water management problems. Indeed, water management has been a continuing and vexatious problem when hydrophobic electrodes are used in conjunction with a proton exchange membrane. In order to supply sufficient humidification for the membrane, water must be condensed onto the hydrogen-side, and sometimes, oxidant side electrode through the use of a separate complex subsystem. Moreover, because hydrophobic electrodes contain a significant amount of Teflon.RTM., which is an electronic insulator, their electronic resistance can be high.
Four different approaches have been tried in order to address a three-phase boundary extension problem. A traditional approach, which has been used in ionomer membrane fuel cells, includes pressing relatively large quantities of platinum black into the membrane. By using this technique, the three-phase boundary can be extended into a third spatial dimension and higher power per unit area can be achieved. These high platinum loadings are very costly, however. Platinum catalyst loadings of 6 milligrams of platinum per square centimeter of fuel cell stack, i.e., including both the fuel and oxidant side of the cell are not uncommon. In order for ionomer membrane fuel cells to achieve commercial viability, the amount of costly, precious metal catalyst per unit area must be substantially reduced from those levels.
A second approach to extending the three-phase boundary of an ionomer membrane fuel cell has been to use a laser to create tiny cavities in the membrane itself. These cavities serve to make the membrane a threedimensional surface and thereby increase its overall surface area. High platinum loadings appear to be required in this design as well, in order to take advantage of the increased surface area and to extend the three-phase boundary into the cavities.
A third approach has been to extend the three-boundary into the body of a pre-catalyzed, hydrophobic electrode by using liquid Nafion.RTM.. The liquid Nafione200 flows into the body of the electrode, thereby extending the three-phase boundary. The electrode itself contains very small quantities of platinum catalyst. The low catalyst loading is possible because the electrode serves as an electronically conductive support for the catalyst. Unfortunately, because the electrode is hydrophobic, its performance suffers because of electronic resistance within the electrode and humidification difficulties.
The fourth, and most recent approach, has been to mix pre-catalyzed carbon particles with an ionomer in liquid suspension (such as liquid Naflon) and to hot press the mixture onto the membrane together with a hydrophobic, Prototech-type electrode or wet-proofed carbon paper. See, e.g., Wilson, U.S. Pat. No. 5,234,777 and the Matsushita electrode described in the Journal of Electrochemical Society, Vol. 142, p. 463 (Feb., 1995).
Thus, state-of-the-art fuel cell electrodes, when used in conjunction with an ionomer membrane, have three principal deficiencies. First, the electrodes rely upon a pressure contact between carbon particles in a binder for electrical conductivity. Even with a graphite cloth backing, there is significant electrical resistance within the electrode because of this type of binding reliance. Second, maintaining humidification of the membrane can be difficult and may require an elaborate subsystem. Third, depending on the design, high levels of platinum catalyst may be required for effective operation.